1. Field of the Invention
This invention relates to an oil-containing resin and its composition, and more particularly to an oil-containing resin capable of producing moldings with improved sliding characteristics and its compositon.
2. Description of the Prior Art
In order to improve the sliding characteristics of thermoplastic resins for use as a sliding material, a method for adding thereto a solid lubricant typical of tetrafluoroethylene resin powder has been widely used. On the other hand, an oil-containing resin comprising a thermoplastic resin and a liquid oil has been also used. In the latter case of oil-containing resin, a mixture of a matrix resin and a liquid oil idly slips at the hopper inlet of a screw extruder owing to the oil attached to the matrix resin and is not bitten into the extruder when it is to be kneaded in the extruder, and substantially no oil-containing resin can be obtained. Furthermore, the conventional oil-containing resin has various problems on the workability, moldability, physical properties of moldings, etc., as mentioned in Japanese patent applicatin Kokai (Laid-open) No. 51-6243, and it now keenly desired to solve these problems.
Another known example of specific oil-containing resin is a porous resin prepared by sintering polyethylene of ultra-high molecular weight, e.g. an average molecular weight of 500,000 or more, and impregnated with a lubricating oil. This oil-containing resin is used as a lubricating material by mixing it with various thermoplastic resins (Japanese Patent Publication No. 47-29374). However, the lubricating materials containing the polyethylene of ultra-high molecular weight have the following disadvantages:
(1) The polyethylene of ultra-high molecular weight has a melting point of 135.degree. to 136.degree. C., and thus cannot be used at a higher temperature than the melting point. PA0 (2) The polyethylene of ultra-high molecular weight has the following thermal behavior: the particles, melt at about 190.degree. C. and adhere to one another and are brought into a sintered state, and undergo abrupt thermal decomposition at 250.degree. C. or higher. Thus, when the oil-containing, sintered polyethylene powder is kneaded into a molding material matrix by a screw extruder at a temperature of 190.degree. C. or higher, the sintered resin melts and is liable to undergo deformation, so that the oil adsorbed in the sintered resin will be dispersed directly into the molding material matrix from the deformed sintered resin. PA0 (3) Important factors that can give sliding characteristics to the oil-containing resin are an amount of oil oozing from the resin onto the sliding surface and a duration of the oozing. When fine polyethylene particles of ultra-high molecular weight having a substantially spherical shape are ideally sintered, the pore capacity will be at most about 40% by volume (=0.4 ml/g), and actually pore capacities of less than 30% by volume are shown in Examples of the said Japanese Patent Publication No. 47-29374. Thus, the amount of the oil to be held by impregnation, that is, the amount of oil to ooze, is limited.
Furthermore, polyamide, polyacetal, etc. have no affinity to oil, as described in the said Japanese Patent Publication No. 47-29374, and thus when the oil-containing, sintered polyethylene powder is kneaded into the polyamide, polyacetal, etc. at a temperature of 190.degree. C. or higher, the oil separated from the sintered powder is dispersed into the polyamide, polyacetal, etc. having no affinity to the oil, and no desired lubricating material can be obtained.
As a result of extensive studies of a method for improving the sliding characteristics of thermoplastic resin without deteriorating the workability, moldability, etc., and without bringing about the said disadvantages of the oil-containing, sintered polyethylene of ultra-high molecular weight, the present inventors have found that an oil-containing resin comprising porous styrene-divinylbenzene copolymer and a liquid oil adsorbed on the copolymer can largely improve the sliding characteristics of a thermoplastic resin as admixed.